When a voltage is applied to a treatment object (the treatment object is anodized) in an acidic solution with the treatment object as an anode, an anodized coating having nanoscale-sized pores is formed.
For example, when an aluminum substrate is anodized in an acidic electrolyte such as sulfuric acid, oxalic acid and phosphoric acid, a porous anodized coating is formed (see Non-Patent Document 1, etc.). The characteristic of this porous coating is that it has a specific geometric structure in which very small columnar pores (alumina holes) with the diameter of several nm to several hundreds of nm are arranged in parallel with the space of several tens of nm to several hundreds of nm. The columnar pore has a high aspect ratio and is excellent in uniformity of diameters of cross sections.
The structure of the porous coating can be controlled to some extent by changing conditions for anodization. For example, it is known that to some extent, the pore-to-pore space can be controlled with the anodization voltage, the depth of the pore can be controlled with anodization time, and the pore diameter can be controlled by pore-widening treatment. The pore-widening treatment is an etching treatment of alumina, and a wet etching treatment with phosphoric acid is usually used.
A method of carrying out two-step anodization for improving verticality, linearity and independency of pores of a porous coating is known. That is, a method of temporarily removing a porous coating formed by anodization, and then carrying out anodization again to form a porous coating having pores having better verticality, linearity and independency is proposed (see Japanese Journal of Applied Physics, Vol. 35, p. 126-129 (1996)). This method uses the fact that dents of an aluminum substrate formed at the time of removing an anodized coating formed by first anodization become pore formation starting points in second anodization.
A method using a stamper having protrusions for forming pores arranged in a desired pattern with high regularity is known (see Japanese Patent Application Laid-Open No. H10-121292; and Masuda, Kotai-Butsuri (Solid Physics), 31, p. 493 (1996)). In this method, the stamper is pressed against the surface of an aluminum substrate to transfer protrusions of the stamper to the surface of the aluminum substrate as dents, whereby pore formation starting points in anodization are prepared.
Nanostructures formed naturally, i.e., formed in a self-regulating manner, as described above, have a potential for achieving a fine and specific structure exceeding conventional artificial nanostructure techniques such as photolithography, electron beam exposure and X-ray exposure, and thus have received enormous attention in recent years.
Particularly, it is believed that by combining techniques for arranging pores regularly, techniques for filling metals, semiconductors and the like in pores, and the like, various nanodevices such as magnetic recording media, magnetic sensors, EL light emitting devices and electroluminescence devices can be realized, and numerous studies are conducted.
In general, aluminum substrates have heavy irregularities, which may cause disturbances and defects in shapes of pores formed. Therefore, the substrates are often subjected to a surface treatment, such as electrolytic polishing. Deposited aluminum films often yield protrusions called hillocks, and have heavy irregularities due to grain boundaries, which may cause disturbances and defects in shapes of pores formed.
However, since it is difficult to sufficiently flatten the surface of the aluminum substrate by electrolytic polishing, and a considerable thickness of aluminum is consumed, it is difficult to provide a thickness sufficient for electrolytic polishing in the case of deposited aluminum films.
Particularly, in the case where highly regulated pores are formed using a stamper, some of the protrusion portions of the stamper are not transferred due to raised portions, such as hillocks. As a result, it may be impossible to form highly regulated pores uniformly. That is, as shown in the schematic diagram of the cross-section of a sample in FIGS. 1A and 1B, some protrusions 13 of the stamper 12 are not transferred as dents 14 due to hillocks 11 existing on the surface of the aluminum film 10. If anodization is carried out in this case, pores are randomly generated from an area 15 where dents are not transferred, resulting in formation of pores 20 having a partly disturbed arrangement 22 in comparison with orderly arrangement 21, as shown in the schematic diagram of the plane of a sample in FIG. 2.
If a metal or semiconductor is to be filled in pores obtained by anodization, various kinds of deposition methods such as a vapor deposition method and a CVD method, but an electrodeposition method is preferable in terms of capability of filling the material in pores having a high aspect ratio, and a Cu or precious metal layer is provided below an anodized coating as an electrode layer for electrodeposition.
However, if such an electrode layer is used, the strength of bonding between the electrode layer and the anodized coating is so weak that it is difficult to form pores extending through the electrode. That is, if anodization is carried out until the bottom of the pore reaches the electrode layer, the anodized coating may fall off. Thus, anodization is stopped before the pore reaches the electrode layer, and a partition wall called a barrier layer existing on the bottom of the pore is removed by chromic-acid based etching, or the like. In this case, there are some variations in the depth of the pore, and therefore it is difficult to stop anodization with good reproducibility with the barrier layer left uniformly over the entire area.
The present invention solves the above problems, and provides an anodized coating suitable for formation of highly regulated pores.